Plasma processing apparatus

ABSTRACT

A plasma processing apparatus includes a vacuum chamber, a sample table that places the sample in the vacuum chamber, and a gas supply unit faced to the sample table and having a gas supply surface with a diameter larger than that of the sample, wherein gas injection holes each having identical diameter are provided concentrically on the gas supply surface, a hole number density of the gas injection holes present in an outer diameter position of the sample or in an outside of the outer diameter position is made higher than that of the gas injection holes present inside the outer diameter position of the sample, and a diameter of the gas injection holes present in the outer diameter position of the sample or in the outside from the outer diameter position is larger than that of the gas injection holes present inside the diameter of the sample.

BACKGROUND OF THE INVENTION

The present invention relates to a plasma processing apparatus to manufacture semiconductor devices, and in particularly to a dry etching technique to etch semiconductor materials, such as a silicon, a silicon dioxide film, etc., along a mask pattern shape formed by a resist material etc.

The dry etching is a semiconductor micro-fabrication method in which a processing gas is introduced into a vacuum chamber having a vacuum decompression unit, the processing gas is turned into a plasma by an electromagnetic wave to apply it to a sample to be processed, a surface of the sample other than a mask portion is etched to obtain a desirable shape. A processing uniformity on an in-plane sample is affected by a plasma distribution, a temperature distribution on the in-plane of the sample, a supplied gas composition and flow rate distribution, etc.

Particularly, in the case of a parallel plate type plasma processing apparatus, the processing gas is supplied from a shower plate disposed so as to face the sample, and a gas supply distribution of the gas supplied from the shower plate has an effect on a process speed, a process shape, etc., since a distance between the sample and the shower plate is relatively short.

As to using the above-mentioned characteristic, JP-A-2006-41088 (corresponding to U.S. patent publication Nos. 2006/16559 and 2007/186972) has proposed a plasma processing apparatus which controls independently the gas composition and flow rate at a center portion and a periphery portion of the shower plate, enhancing the in-plane uniformity of the sample, such as a process shape.

FIG. 7 shows a shower plate as related art.

Normally, the shower plate has been designed that a plurality of gas injection holes 2 are uniformly disposed on a shower plate gas supply surface 5, such that the gas composition and flow rate injected from every hole should be uniformed and a gas supply condition applied per unit area of the sample is also uniformed, basically.

Further, the gas supply amount is broadly controlled at the center portion and periphery portion of the in-plane sample to cancel an effect caused by a reactive product etc., realizing the uniformity of the processed shape.

In the case of a gas supply distribution structure disclosed in the JP-A-2006-41088, the gas composition and flow rate injected from every hole are different in the two domains: the center portion and the periphery portion, but the gas having the same gas composition and flow rate is injected from the holes present in the respective domains.

SUMMARY OF THE INVENTION

There is a tendency for the gas supply amount at the periphery portion of the sample to relatively go down compared with the center portion and its vicinity thereof, since the gas injection holes to be formed on the shower plate are basically disposed in uniformity.

Particularly, in the case of a narrow-gap type apparatus, there sometimes arises a problem to occur a non-uniformity shape at the periphery portion of the sample by causing the non-uniformity of gas supply amount.

FIG. 3 shows a relation of an aspect ratio (D/L) and a relative molecule flux, where a wafer (sample) diameter is D(300 mm), and a distance from the wafer to the shower plate is L. This is a result in which a relative amount of the gas molecules reached to the faced wafer is calculated by one dimension, in the case where it assumes that the gas molecules injected uniformly from the gas injection holes of the shower plate are isotropically diffused, and it also assumes that the shower plate having the gas injection holes faced to the wafer has the same diameter and the number of holes per unit area is uniformity.

As shown in FIG. 3, it is appreciated that the relative amount of the gas molecules reached to the wafer surface is relatively deficient at the wafer periphery portion when the aspect ratio becomes large. That is, it has become clear that the relative deficiency of the gas supply amount at the edge portion of the wafer occurs from a condition where the distance between the wafer and shower plate is equal to or less than 300 mm, where the aspect ratio is equal to or greater than 1, or the wafer diameter is 300 mm (φ300 mm).

As to a solution method for the problem indicated on FIG. 3, it is possible to be thought of a method such that a domain of the gas injection holes formed on the shower plate is expanded in relation to the diameter of sample.

FIG. 4 shows a relation between a gas injection domain diameter and the relative molecule flux.

This is a result of the case where the wafer diameter is 300 mm, the gas injection holes are disposed uniformly on the shower plate, and the distance L between the wafer and shower plate is 24 mm (aspect ratio D/L=12.5).

As shown in FIG. 4, for a purpose of obtaining a sufficient gas supply uniformity in this method that expands the diameter of the gas injection hole domain, it has become clear that the gas injection hole domain diameter requires about 1.5 times the wafer diameter D, that is, the gas injection hole domain diameter is set substantially to equal to or greater than 450 mm (φ450 mm).

In fact, since the expansion of the gas injection domain diameter incurs a large size apparatus caused by a large-sized shower plate and the shower plate is normally exchanged regularly as a consumable supply, the cost of the consumable supply increases by causing the large size, as a problem, and the expansion is not helpful to practically solve the problem.

An object of the invention is to solve the gas supply deficiency occurred at the periphery portion of the sample when the gas is supplied from the shower plate, and to provide a plasma processing apparatus capable of enhancing the in-plane uniformity of processing accuracy on the sample.

Particularly, the invention is to provide a plasma processing apparatus having both enhancement of the in-plane uniformity of the sample in the processing characteristic and cost reduction of the consumable supply by restraining the expansion of the shower plate diameter in minimum and improving the gas supply uniformity to the in-plane sample.

According to one aspect of the invention to solve the problem, a plasma processing apparatus for applying a surface processing to a sample, includes a vacuum chamber, a sample table to place the sample in the vacuum chamber, and a gas supply unit faced to the sample table and having a gas supply surface with a diameter larger than that of the sample, in which gas injection holes each having identical diameter are provided concentrically on the gas supply surface of the gas supply unit, and a hole number density of the gas injection holes present in an outer diameter position of the sample or in an outside of the outer diameter position is made higher than that of the gas injection holes present inside the outer diameter position of the sample.

According to another aspect of the invention, a plasma processing apparatus for applying a surface processing to a sample, includes a vacuum chamber, a sample table to place the sample in the vacuum chamber, and a gas supply unit faced to the sample table and having a gas supply surface with a diameter larger than that of the sample, in which gas injection holes are provided concentrically on the gas supply surface of the gas supply unit, and a diameter of the gas injection holes present in an outer diameter position of the sample or in an outside from the outer diameter position is larger than that of the gas injection holes present inside from the outer diameter position of the sample.

According to the invention, a uniformed gas supply distribution is given to the entire surface of the sample without making the apparatus large and also making the shower plate large as a change part, realizing the uniformity of processing rate and processing shape of the sample.

The other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view of a plasma processing apparatus in the invention;

FIG. 2 is a schematic view of a shower plate in a first embodiment of the invention;

FIG. 3 is an explanatory diagram of a relative molecule flux distribution on a wafer surface in a condition obtained from a ratio (D/L) where a wafer diameter D and a distance L between the wafer and the shower plate;

FIG. 4 is a diagram showing an effect of a gas injection domain diameter in relation to the wafer diameter;

FIG. 5 is a diagram for explaining an advantage of the invention;

FIG. 6 is a diagram for explaining an advantage of the invention;

FIG. 7 is a schematic view of a related shower plate;

FIG. 8 is a diagram showing the relative molecule flux on the wafer surface when a gas-injection-hole number density is made increased at 280 mm (φ280 mm) and its vicinity of the shower plate;

FIG. 9 is a diagram showing the relative molecule flux on the wafer surface when the gas-injection-hole number density is made increased at 290 mm (φ290 mm) and its vicinity of the shower plate;

FIG. 10 is a diagram showing the relative molecule flux on the wafer surface when the gas-injection-hole number density is made increased at 300 mm (φ300 mm) and its vicinity of the shower plate;

FIG. 11 is a diagram showing the relative molecule flux on the wafer surface when the gas-injection-hole number density is made increased at 320 mm (φ320 mm) and its vicinity of the shower plate;

FIG. 12 is a diagram showing the relative molecule flux on the wafer surface when the gas-injection-hole number density is made increased at 330 mm (φ330 mm) and its vicinity of the shower plate;

FIG. 13 is a diagram showing the relative molecule flux on the wafer surface when the gas-injection-hole number density is made increased at 340 mm (φ340 mm) and its vicinity of the shower plate;

FIG. 14 is a diagram showing the relative molecule flux on the wafer surface when the gas-injection-hole number density is made increased at 360 mm (φ360 mm) and its vicinity of the shower plate; and

FIG. 15 is a schematic view of the shower plate in a second embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the invention will be described with reference to the drawings.

Embodiment 1

A first embodiment of the invention will be described with use of FIG. 1 and FIG. 2.

FIG. 1 is shows a section view of a plasma processing apparatus in one embodiment of the invention. The plasma processing apparatus includes an electrostatic chucking function built-in electrode (sample table) 15 for placing a sample 7 in a vacuum chamber 24 and a shower plate (gas supply unit) 1 faced to the sample table 15. In this way, a 200 MHz high-frequency power is supplied from a discharge-use high-frequency power source 13 to a conductor-type antenna 12 incorporated with a plate 8 and a dispersion plate 11 to turn a gas supplied from the shower plate 1 into a plasma in a discharge space 14. Further, a 4 MHz high-frequency voltage is applied to the sample 7 from a high-frequency power source 16 via the electrostatic chucking function built-in electrode 15 to accelerate ions in the plasma and to be incident to the surface of the sample 7. The 4 MHz high-frequency voltage is also applied independently to the antenna 12 from a high-frequency power source 17 by superimposing with a discharge-use 200 MHz high-frequency power, so that an ion energy in the plasma incident to the surface of shower plate 1 is controlled independently from a plasma generation and a bias condition of the sample. The antenna 12 and electrostatic chucking function built-in electrode 15 are also controlled respectively in temperature by insulation type liquid cooling circulating functions 21, 22.

The shower plate 1 is formed by silicon. The plate 8 is disposed on an upper stage of the shower plate 1, and the plate 8 has holes matched with the same position of gas injection holes 2 formed on the shower plate 1 and slightly larger than the gas injection holes 2 in diameter. The dispersion plate 11 is further disposed on the upper stage of the plate 8, and the dispersion plate 11 forms a gas dispersion layer 10 to disperse the gas supplied from a gas supply portion 9. The gas supply portion 9 is provided independently for an inside domain and an outside domain of the sample 7, and a flow rate and a gas composition can be controlled independently at the inside and outside domains of the sample 7. The inside domain and outside domain are also divided by a barrier in such that a form domain area of the respective gas injection holes 2 in the inside and outside domains is substantially equal. In the case of this embodiment, the apparatus will be described with two domains: the inside domain and outside domain, and the domain may not be divided, but also divided into more than three domains. In addition, reference numerals 18, 19 and 20 denote an automatic matching device, 6 denotes a shower plate fixing screw hole, 23 denotes a silicon-made focus ring, 25 denotes an insulation material, and 27 denotes an earth plate.

In the case of FIG. 1, a silicon wafer of 300 mm in diameter is used for the sample. The gas injection holes 2 formed on the shower plate 1 are formed within a range of 314 mm (φ314 mm) in diameter, the inside of which is the inside domain of 200 mm (φ200 mm), and the outside of which is the outside domain. The gas dispersion layer 10 is also formed independently for the inside and outside domains such that the gas is dispersed uniformly in the respective inside and outside domains.

FIG. 2 shows a layout of the gas injection holes 2 on the surface of shower plate 1, in which a diameter of the gas injection hole 2 is 0.5 mm, and a thickness of the domain where the gas injection holes 2 are formed on the shower plate 1 is 10 mm. The diameter of the gas injection holes 2 formed on a shower plate gas supply surface 5 is all the same. The gas injection holes 2 are also formed in concentricity and in an equal interval (10 mm pitch) from a shower plate center 3. The number of gas injection holes formed on the circumferences is substantially proportional to the circumference from the center to the periphery and its vicinity. Therefore, the number of gas injection holes per unit area on the shower plate 1 is substantially the same in the layout, from the center to the periphery and its vicinity. The diameter of shower plate gas supply surface 5 is made larger than that of the sample 7.

In the case of the constitution in FIG. 2, the total number of gas injection holes of the outside domain in the periphery domain is about twice that of the inside domain. Therefore, the gas is flown into the outside domain by the flow rate having about twice that of the inside domain, so that the gas flow rate injected from every gas injection hole becomes equal at both the inside and outside domains.

According to the above-mentioned constitution, the gas injected from the gas injection holes 2 is substantially the same in the flow rate and gas composition at the inside and outside domains of the sample 7. A gas condition (flow rate and composition) distribution produced by supplying the gas to the surface of the sample 7 depends on a density of number of the gas injection holes 2. In the case of this embodiment, the apparatus will be described with a case where the gas flow rate injected from every gas injection hole 2 is equal. However, it is not necessarily to make the gas flow rate equal, injected from every gas injection hole 2, since an oxygen flow rate is sometimes changed at the inside and outside domains, for example, for a purpose of correcting a deposition distribution caused by a reactive product.

In the case of this embodiment, as to a position corresponding to an edge portion of the sample 7, a hole number density per unit length on two outermost circumferences formed with the gas injection holes 2 is set to about twice that of the other circumferences. A pitch between the gas injection holes 2 formed on the other circumferences is 10 mm, while the pitch between the holes 2 formed on the two outermost circumferences is 7 mm.

In consequence, the hole number density of the gas injection holes 2 facing to the edge portion of the sample 7 increases by about 2.85 times (density (twice) of circumferential direction×density (10 mm/7 mm) of diametrical direction), compared with the other domains.

That is, a uniformity gas supply is carried out at the inside domain of the sample 7 since the gas injection holes 2 are disposed on the inside domain with an equal density, however, a large volume gas, much more than the other domains, is supplied to the edge portion of the sample 7 at the outside domain since the density of the gas injection holes 2 formed on the edge portion of the sample 7 is high.

FIG. 5 shows a calculation result of the relative molecule flux at the wafer edge portion in the case of the shower plate in the invention and the shower plate as related art.

FIG. 5 shows the case where a wafer diameter D is 300 mm (φ300 mm) and a distance L between the wafer and the shower plate is 24 mm (aspect ratio D/L=12.5).

As shown in FIG. 5, it can be confirmed that a gas supply amount deficiency is made up for the wafer edge portion and its vicinity to supply uniformly the gas to the entire wafer surface, in the case of the shower plate 1 of the invention. On the other hand, the gas supply amount is relatively short at the wafer edge portion and its vicinity, compared with the center portion of the wafer, in the case of the related shower plate. This is assumed that an exhaust velocity becomes fast at a long circumference wafer edge portion, compared with the center portion of the wafer, when the gas supplied from the shower plate 1 is exhausted from the periphery of the wafer. The gas injected from the periphery portion reaches to a center domain as isotropically diffused, however, it is assumed that there is no gas supply from the outside other than the outermost circumference formed with the gas injection holes 2.

In this way, by using the shower plate 1 in the invention, it is possible that the gas is supplied uniformly, therefore, it has become clear that the shower plate 1 is useful to make an etching characteristic uniformed.

Particularly, as used with a narrow-gap type opposite electrode structure, in the case of an etching mechanism (a silicon dioxide film etching by using a phlorocabon-based gas etc.) of which the etching characteristic depends largely on the supplied gas flow rate rather than a gas pressure, a difference of an etching rate and etching shape can be restrained within the in-plane wafer.

FIG. 6 shows an etching rate distribution of a TEOS film in the case of the shower plate 1 in the invention and the shower plate as related art.

In the case of using the shower plate 1 of the invention, the gas flow rate of the outside domain is set to about twice that of the inside domain, that is, an inside flow rate is set to Ar=500 sccm, C₄F₈=15 sccm, O₂=15 sccm, and an outside flow rate is set to Ar=1000 sccm, C₄F₈=30 sccm, O₂=30 sccm, in accordance with a gas injection hole number ratio (about twice), since the gas supply amount injected from every gas injection hole 2 is made equal for all of the holes 2 formed on the inside and outside domains.

On the other hand, in the case of using the related shower plate, the same gas flow rate is supplied to both the inside and outside domains, that is, the inside and outside flow rates are of an Ar/C₄F₈/O₂ mixed gas containing Ar=500 sccm, C4F8=15 sccm, O2=15 sccm, since the number of the gas injection holes at the inside domain is substantially equal to that of the outside domain.

As shown in FIG. 6, in the case of using the related shower plate, the etching rate at the wafer edge portion is lowered, and an etching rate uniformity is as much as 8%. In the case of using the shower plate 1 of the invention, no effect is given to the etching rate at a wafer center domain, the etching rate at the wafer edge portion is increased, and the etching rate uniformity is improved to as much as 3%.

In the case of the invention, it is possible to select an optimal gas supply distribution in response to processing objects and processing conditions, by changing the gas-injection-hole number density so as to adapt the etching characteristic.

Next, the following description will be concerned with an optimization for the gas-injection-hole number density and an optimization for the domain on which the gas-injection-hole number density is made increased.

FIG. 8 shows a calculation result of the relative molecule flux on the wafer surface in the case where the gas-injection-hole number density is increased at 280 mm (φ280 mm) and its vicinity of the shower plate 1.

FIG. 9 shows a calculation result of the relative molecule flux on the wafer surface in the case where the gas-injection-hole number density is increased at 290 mm (φ290 mm) and its vicinity of the shower plate 1.

FIG. 10 shows a calculation result of the relative molecule flux on the wafer surface in the case where the gas-injection-hole number density is increased at 300 mm (φ300 mm) and its vicinity of the shower plate 1.

FIG. 11 shows a calculation result of the relative molecule flux on the wafer surface in the case where the gas-injection-hole number density is increased at 320 mm (φ320 mm) and its vicinity of the shower plate 1.

FIG. 12 shows a calculation result of the relative molecule flux on the wafer surface in the case where the gas-injection-hole number density is increased at 330 mm (φ330 mm) and its vicinity of the shower plate 1.

FIG. 13 shows a calculation result of the relative molecule flux on the wafer surface in the case where the gas-injection-hole number density is increased at 340 mm (φ340 mm) and its vicinity of the shower plate 1.

FIG. 14 shows a calculation result of the relative molecule flux on the wafer surface in the case where the gas-injection-hole number density is increased at 360 mm (φ360 mm) and its vicinity of the shower plate 1.

As shown in FIG. 8 and FIG. 9, the gas-injection-hole number density is increased to thereby increase the gas supply amount at the wafer edge portion in the inside domain inside the wafer diameter or the wafer outer diameter position. This causes the gas supply amount to increase the inside, but it has become clear that the gas supply distribution is slightly improved.

On the other hand, as shown in FIG. 10 to FIG. 14, the gas-injection-hole number density is increased to increase the gas supply amount at the wafer edge portion in the domain of the wafer diameter, that is, the wafer outer diameter position, or the domain outside the wafer outer diameter position. It has become clear that the uniformity gas supply distribution is obtained in the wafer domain.

However, as shown in FIG. 13 and FIG. 14, in the case where the gas injection hole is added to the domain at 340 mm (φ340 mm) or more, it is necessary to also increase the gas supply amount since a necessary increased number caused by the additional position gas-injection-hole number density becomes equal to or greater than 4 times. Therefore, the apparatus is subject to an increase of gas consumed amount and an increase of the strain on the exhaust performance.

In consequence, as shown in FIG. 10 to FIG. 12, it is desirable to increase the gas-injection-hole number density in a range of about 300 mm (φ300 mm) to 330 mm (φ330 mm), that is, as much as 1.0 to 1.1 times the wafer diameter.

Further, the increase of the gas-injection-hole number density varies in response to the processing objects and processing conditions. However, the gas-injection-hole number density increases in the range of 1.5 to 4 times to thereby optimize the uniformity of the etching characteristic, and the gas consumed amount can be restrained.

Embodiment 2

A second embodiment of the invention will be described with use of FIG. 15.

FIG. 15 is a schematic diagram showing a shower plate 1 in the second embodiment of the invention.

In the case of this embodiment, each diameter of gas injection holes 27 faced to the wafer edge portion and formed on the periphery portion of the shower plate 1 is 1.3 times that of the other gas injection holes 2, that is, the hole diameter at the periphery portion is 0.65 mm while the other hole diameter is set to 0.5 mm, and the gas-injection-hole number density is set to uniformity. In the case of the first embodiment, the gas supply amount to the wafer edge portion is adjusted by the gas-injection-hole number density of the gas injection holes 4 each having the same diameter and formed at the periphery portion of the shower plate 1. In the case of the second embodiment, the gas supply amount is adjusted by the hole diameter.

A conductance at a time when the gas passes through the gas injection holes 2 of the shower plate 1 increases in proportion to the 3 to 4 power of the hole diameter (3 power in the case of molecule flow, and the 4 power in the case of viscous flow). Practically, the conductance becomes a middle value (the 3.5 power in a middle flow) between the molecule flow and the viscous flow.

Therefore, it is possible to obtain the same effect as increased the gas-injection-hole number density by expanding the hole diameter, even in the same gas-injection-hole number density.

In the case of the second embodiment, the gas-injection-hole number density is the same at the periphery portion and the other portion, and the hole diameter of the periphery portion is 1.3 times that of the other portion, so that the gas supply amount at the periphery portion can be enhanced by about 2.85 times.

As with the first embodiment, the expansion amount of the hole diameter can be changed by the processing objects and processing conditions. For a purpose of increasing the gas-injection-hole number density from 1.5 to 4.0 times, that is, increasing the gas supply amount from 1.5 to 4.0 times, the hole diameter is set to a range from 1.1 times (1/3.5 power of 1.5=1.123) to 1.5 times (1/3.5 power of 4=1.486), so that the uniformity of the etching characteristic can be optimized.

Further, the domain on which the gas injection hole diameter is expanded can be ranged desirably from 1.0 to about 1.1 times, which is similar to the first embodiment.

The invention relates to a semiconductor device manufacturing apparatus, and in particularly to a plasma etching apparatus to apply an etching processing to a semiconductor material masked with a pattern drawn by the lithography technique. According to the invention, it is possible to enhance the processing characteristic at the silicon wafer edge portion as a sample, particularly, the uniformity of the processing rate and processing shape. From the above-mentioned advantages of the invention, a non-defective product acquired rate is enhanced for the silicon wafer edge portion, and a processing yield of the etching apparatus can be enhanced.

It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims. 

1. A plasma processing apparatus for applying a surface processing to a sample, comprising: a vacuum chamber; a sample table that places the sample in the vacuum chamber, and a gas supply unit faced to the sample table and having a gas supply surface with a diameter larger than that of the sample, wherein gas injection holes each having identical diameter are provided concentrically on the gas supply surface of the gas supply unit, and a hole number density of the gas injection holes present in an outer diameter position of the sample or in an outside of the outer diameter position is made higher than that of the gas injection holes present inside the outer diameter position of the sample.
 2. The apparatus according to claim 1 wherein the hole number density of the gas injection holes present in an outer diameter position of the sample or in an outside of the outer diameter position is present in a range from 1.5 to 4.0 times that of the gas injection holes present inside the outer diameter position of the sample.
 3. The apparatus according to claim 1 wherein the gas injection holes present in the outer diameter position of the sample or in the outside of the outer diameter position are present in a range of 1.0 to 1.1 times the diameter of the sample.
 4. The apparatus according to claim 1 wherein an aspect ratio (D/L) is equal to or greater than 2, where the diameter of sample is D, and a distance from the sample to the gas supply surface is L.
 5. A plasma processing apparatus for applying a surface processing to a sample, comprising: a vacuum chamber; a sample table that places the sample in the vacuum chamber; and a gas supply unit faced to the sample table and having a gas supply surface with a diameter larger than that of the sample, wherein gas injection holes are provided concentrically on the gas supply surface of the gas supply unit, and a diameter of the gas injection holes present in an outer diameter position of the sample or in an outside from the outer diameter position is larger than that of the gas injection holes present inside from the outer diameter position of the sample.
 6. The apparatus according to claim 5 wherein a diameter of the gas injection holes present in the outer diameter position of the sample or in an outside of the outer diameter position is present in a range of 1.1 to 1.5 times that of the gas injection holes present inside the outer diameter position of the sample.
 7. The apparatus according to claim 5 wherein the gas injection holes present in the outer diameter position of the sample or in the outside of the outer diameter position are present in a range of 1.0 to 1.1 times the diameter of the sample.
 8. The apparatus according to claim 5 wherein an aspect ratio (D/L) is equal to or greater than 2, where the diameter of sample is D, and a distance from the sample to the gas supply surface is L. 